This invention relates to electrical devices having arrays of pixels, for example liquid crystal displays or optical image sensors. The invention is particularly concerned with devices having arrays of pixels which are divided into sub-arrays.
In order to make possible the manufacture of large area arrays, it is known to form a device from a plurality of sub-arrays of pixels which are arranged beside each other in order to provide a large composite array. The known problem arises, both for image sensors and for displays, that it is desirable to disguise the join between the sub-arrays. Generally, this may be achieved in one of two ways. One possibility is to use sub-arrays which may be abutted together, whilst maintaining a uniform pixel pitch across the join. Obviously, this requires very accurate machining of the sub-arrays, particularly when the pixel dimensions are to be reduced to a minimum size, to increase the resolution of the device. Also, electrical connections to peripheral circuitry can not be made along abutting edges, restricting the number of sub-arrays that can be used. The alternative possibility is to employ an optical system, which enables the sub-arrays to be slightly spaced apart. The present invention concerns an optical solution to the problem of tiling a plurality of sub-arrays to enable the manufacture of a larger array.
The basic principle underlying the use of an optical system to enable tiling of sub-arrays is that the optical system should provide an optical enlargement of each sub-array, so that the image of the sub-arrays produced by the optical system constitutes a uniform image, and the joins between the sub-arrays have been eliminated. In other words, in the case of a display, an enlarged view of the sub-arrays is viewed, and in the case of an image sensor, the image to be sensed is divided into portions which are reduced in size for sensing by the sub-arrays.
This basic concept of optical enlargement could, for example, suggest the use of the arrangement employed in a Galilean telescope, as represented schematically in FIG. 1. Three sub-arrays are represented as 10, 20 and 30, each having an associated diverging lens 11, 21, 31 and an associated converging lens 12, 22, 32. Such an arrangement can maintain parallel beams at the output of the converging lenses, whilst providing enlargement. However, the image seen from the viewing area VA is an enlarged virtual image of each sub-array 10, 20, 30. For a display, this presents the problem that the range of viewing angles may be limited, because viewing from different angles will result in some loss of the image at the boundaries, since these images are virtual and may be partially hidden by the lens apertures. In the case of an image sensor, imaging must take place at the real optical focus of the optical system, so the arrangement of FIG. 1 will only work if a supplementary optical system is provided to image the document at the virtual image plane.
U.S. Pat. No. 4,321,628 discloses an image scanning system in which a linear array of photosensitive elements is divided into sub-arrays which are spaced apart. Each sub-array is associated with a pair of converging lenses which effectively focus a non-inverted image of the sub-array onto a platen 10, where a document can be placed for scanning. The sensor arrangement of U.S. Pat. No. 4,321,628 requires converging lenses of sufficient power to produce a focused image of each entire sub-array at the platen, and this imposes a significant distance between the array and the document to be sensed. Size constraints are, of course, equally important in the case of displays where the overall depth of a display panel is to be maintained as small as possible.